This invention relates to a heat treatment of a nickel base alloy to produce an article exhibiting an acceptable level of grain boundary precipitates, reduced residual stress, with an optimum balance of tensile, stress rupture and creep properties. The invention has particular utility in the production of components for gas turbine and jet engines, such as turbine discs.
For the compositions hereinafter defined, heat treatment steps are maintained within relatively narrow, critical limits which have been found to be necessary to achieve the novel combination of reduced residual stress and optimum mechanical properties, while at the same time effecting a reduction of about 50% in processing time and cost, as compared to a conventional prior art treatment of a nickel base alloy.
So-called "superalloys" which are widely used for components in gas turbine and jet engines include nickel base alloys sold under the trademarks "IN-100" by International Nickel Co., Inc. and "Rene 100" by General Electric Company. The International Nickel Co., Inc. alloy is disclosed in U.S. Pat. No. 3,061,426. According to "Aerospace Structural Metals Handbook Chapter IN-100", by S. S. Manson, Code 4212, 1978 revision, page 6, the composition of IN-100 is as follows:
cobalt 13-17% PA1 chromium 8-11% PA1 aluminum 5-6% PA1 titanium 4.5-5.0% PA1 aluminum plus titanium 10-11% PA1 molybdenum 2-4% PA1 iron 0-1% PA1 vanadium 0.7-1.2% PA1 boron 0.01-0.02% PA1 carbon 0.15-0.20% PA1 manganese 0.10% maximum PA1 sulfur 0.015% maximum PA1 silicon 0.15% maximum PA1 nickel balance PA1 cobalt 14-16% PA1 chromium 9-10% PA1 aluminum 5.3-5.7% PA1 titanium 4.0-4.4% PA1 molybdenum 2.7-3.3% PA1 iron 0-1% PA1 vanadium 0.9-1.1% PA1 boron 0.01-0.02% PA1 carbon 0.15-0.20% PA1 nickel balance
The same literature source indicates the composition of Rene 100 to be as follows:
In this same literature source, introductory comments at page 1 include the following:
"Because of the large quantities of strengthening elements included in the composition, the alloy is not hot worked, and is therefore used in the as-cast condition. Recently, however, there has been considerable development of a powder metallurgy product which permits working of the alloy. At high temperatures the powder consolidated product becomes superplastic, thus opening many possibilities in fabrication-to-shape of wrought complex components.
"Also, because of the high content of gamma prime precipitate that constitutes one of the strengthening components of the alloy, the equilibrium solution temperature approaches the solidus, so the material is usually used in the as-cast condition, without heat treatment. However, it is subjected to heat treatment during the deposition of protective coatings. The powder metallurgy product is heat treated to achieve desirable properties."
It is next pointed out that protective coatings may be needed for high temperature applications due to the relatively low oxidation and corrosion resistance of the alloy. A number of types of coatings such as aluminizing or chromizing have been found to provide sufficient protection. Additionally, precipitation of sigma phase with resulting embrittlement has been found to occur after exposure to high temperature and stress for long periods of time. Restriction of the aluminum plus titanium contents has been found to be effective in minimizing sigma phase formation, and the limitation on the aluminum plus titanium levels is based on electron vacancy density calculations.
Page 1 of this literature source further states:
"For the powder metallurgy product, Pratt and Whitney Aircraft recommends solutioning at 2050.degree. F., stabilization at 1600.degree. and 1800.degree. F., and precipitation hardening at 1200.degree. and 1400.degree. F. Typical heat treatment used . . . 2215.degree. F., 4 hrs+2000.degree. F., 4 hrs+1550.degree. F., 16 hrs."
Data relating to IN-100 are also contained in "Alloy Digest", filing code: Ni-151, March 1970; "Properties of Superalloys/243" and "Guide to Selection of Superalloys", pages 14 and 15, W. F. Simmons et al.
United States Patents relating to nickel base alloys and treatment thereof include U.S. Pat. Nos. 3,653,987; 3,667,938; 4,083,734; 4,093,476; 4,121,950 and 4,253,884.
U.S. Pat. No. 3,653,987, issued Apr. 4, 1972 to W. J. Boesch, discloses an alloy consisting essentially of up to 0.18% carbon, 14.2 to 20% cobalt, 13.7 to 16% chromium, 3.8 to 5.5% molybdenum, 2.75 to 3.75% titanium, 3.75 to 4.75% aluminum, up to 4% iron, 0.005 to 0.035% boron, up to 0.5% zirconium, up to 0.5% hafnium, up to 0.75% columbium, up to 0.5% rhenium, up to 0.75% tantalum, up to 1.0% manganese, up to 3% tungsten, up to 0.5% rare earth metals, and balance essentially nickel with incidental impurities. This alloy is heat treated to develop gamma prime particles consisting essentially of randomly dispersed irregularly shaped particles less than 0.35 micron in diameter. The treatment involves heating at a temperature of at least 2000.degree. F., cooling, and heating at a temperature of about 1500.degree. to about 1850.degree. F. An optional third stage of heat treatment for precipitation hardening may be conducted at 1350.degree. to 1450.degree. F. This patent points out that a prior art heat treatment for nickel base alloys comprised the steps of heating at a temperature of 2135.degree. F. for 4 hours and cooling; heating at a temperature of 1975.degree. F. for 4 hours and cooling; heating at a temperature of 1550.degree. F. for 4 hours and cooling; and heating at a temperature of 1400.degree. F. for 16 hours and cooling.
U.S. Pat. No. 4,083,734, issued Apr. 11, 1978 to W. J. Boesch, discloses a nickel base alloy consisting essentially of from 12.0 to 20.0% chromium, 4.75 to 7.0% titanium, 1.3 to 3.0% aluminum, 13.0 to 19.0% cobalt, 2.0 to 3.5% molybdenum, 0.5 to 2.5% tungsten, 0.005 to 0.03% boron, 0.005 to 0.045% carbon, up to 0.75% manganese, 0.01 to 0.08% zirconium, up to 0.5% iron, up to 0.2% rare earth elements, up to 0.02% of magnesium, calcium, strontium, barium, and mixtures thereof, and balance essentially nickel, with titanium plus aluminum from 6.5 to 9.0%. A maximum carbon level of 0.045% is alleged to increase the hot impact strength of the alloy without adversely affecting stress rupture properties. An exemplary treatment for a wrought alloy of this patent was heating at 2150.degree. F. for 4 hours and air cooling; heating at 1975.degree. F. for 4 hours and air cooling; heating at 1550.degree. F. for 24 hours and air cooling; and heating at 1400.degree. F. for 16 hours and air cooling.
U.S. Pat. No. 4,093,476, issued June 6, 1978 to W. J. Boesch, differs from U.S. Pat. No. 4,083,734 principally in permitting from 0.05 to 0.15% carbon and requiring from 0.031% to 0.048% boron. Carbon within the range of 0.02% to 0.04% and boron within the range of 0.032% to 0.045% are alleged to provide the best combination of stress rupture life and impact strength. An exemplary heat treatment of this patent differed from that of U.S. Pat. No. 4,083 734 only by specifying a first heating step of 2135.degree. F. for 4 hours.
U.S. Pat. No. 4,121,950, issued Oct. 24, 1978 to A. R. Guimier et al, discloses a nickel base alloy consisting essentially of 13 to 20% cobalt, 13 to 19% chromium, 3% to 6% molybdenum, tungsten or mixtures thereof, 0.01 to 0.20% carbon, 2 to 4% aluminum, 0.10 to 3% titanium, 0.30 to 1.50% hafnium and remainder nickel. The heat treatment process is described and claimed functionally as "(a) placing at least a portion of the gamma prime phase back into solution, (b) effecting the coalescence of carbides and the initiation of the reprecipitation of the gamma prime phase, and (c) completing the reprecipitation of the gamma prime phase."The actual steps involve heating at about 1050.degree. to 1200.degree. C. for at least one hour and cooling; heating at about 850.degree. C. for 10 to 30 hours and cooling; and heating at about 760.degree. C. from 10 to 30 hours. Preferably aluminum plus titanium ranges between about 4% and 7% with the ratio of titanium to aluminum about 0.20 to 1.5.
U.S. Pat. No. 4,253,884, issued Mar. 3, 1981 to G. E. Maurer et al, discloses a method of heat treating and incorporating a coating operation therewith for a nickel base alloy consisting essentially of from 12.0 to 20.0% chromium, 4.0 to 7.0% titanium, 1.2 to 3.5% aluminum, 12.0 to 20.0% cobalt, 2.0 to 4.0% molybdenum, 0.5 to 2.5% tungsten, 0.005 to 0.048% boron, 0.005 to 0.15% carbon, up to 0.75% manganese, up to 0.5% silicon, up to 1.5% hafnium, up to 0.1% zirconium, up to 1.0% iron, up to 0.2% rare earth elements, up to 0.1% magnesium, calcium, strontium, barium and mixtures thereof, up to 6.0% rhenium and/or ruthenium, and balance essentially nickel, with titanium plus aluminum being from 6.0 to 9.0% and a titanium to aluminum ratio of 1.75 to 3.5. The heat treatment to which this alloy is subjected comprises heating at a temperature of at least 2050.degree. F., cooling; heating between 1800.degree. and 2000.degree. F., cooling; heating between 1500.degree. and 1800.degree. F.; coating the alloy with a cobalt, nickel or iron base alloy; heating the coated alloy to a temperature of at least 1600.degree. F., cooling; and heating the alloy within the range of 1300.degree. and 1500.degree. F.
It is therefore evident that there are numerous specific compositions within the general class of nickel base superalloys and a variety of heat treatments therefor. All heat treatments of which applicants are aware appear to have in common the objective of placing in solution the gamma prime particles or phase which is composed of M.sub.3 (Al, Ti) wherein M is primarily nickel with relatively minor amounts of chromium and molybdenum. Thereafter the next stage of heat treatment is for the purpose of reprecipitating the gamma prime phase and to form a grain boundary precipitate of metal carbides. The third stage (if practiced) is a precipitation hardening or aging treatment wherein nickel, aluminum and titanium compounds are precipitated. In substantially all the prior art patents discussed above it is pointed out that MC carbides are precipitated in the grain boundaries, with M being principally titanium, molybdenum and/or chromium. Even in U.S. Pat. No. 4,083,734, which limits carbon to a maximum of 0.045%, it is emphasized that carbides are formed and precipitate in the grain boundaries, but it is alleged that the carbon level specified in this patent inhibits transformation in service of MC carbides to M.sub.23 C.sub.6 carbides (wherein M is predominantly chromium), the latter being alleged to be responsible for a loss of hot impact strength.